In order to improve the breakdown voltage and reduce the on-resistance of a lateral metal-oxide-semiconductor transistor (MOST), the conventional reduced surface field (RESURF) technique is typically used. However, it can be inferred from X. B. Chen, et al., “Optimization of the drift region of power MOSFET's with lateral structure and deep junctions,” IEEE Trans. E. D. vol. ED-34, No. 11, pp. 2344-2350 (1987), that the breakdown voltage of a surface device using the RESURF technique can only achieve about 70% of that of a one-sided abrupt parallel-plane junction with the same substrate. Moreover, the specific on-resistance of a lateral MOST using such a technique is large. In commonly-assigned U.S. Pat. Nos. 5,726,469 and 6,310,365 B1, methods for using optimum variation surface doping to implement the surface voltage-sustaining region are disclosed. By using these methods, the breakdown voltage of a surface device can reach 90% of that of a one-sided abrupt parallel-plane junction with the same substrate. Furthermore, lateral MOSTs with a small specific on-resistance of the drift region can be made. Such a surface voltage-sustaining region can be realized by the offset of the effects of the impurities of different located p-regions and n-regions. Therefore, the fabrication can be compatible with conventional CMOS or BiCMOS technology.
In X. B. Chen, et al., “Theory of optimum design of reverse-biased p-n junction using resistive field plates and variation lateral doping,” Solid-State Electronics, Vol. 35, No. 8, pp. 1365-1370 (1992), it has been shown that the optimum surface doping density, e.g., donor density, for a surface voltage-sustaining region on an oppositely doped substrate, e.g., a p−-substrate, has been disclosed. FIG. 1(a) shows the structure of the voltage-sustaining region of an interdigitated layout cell of a diode having a p−-type substrate 1, an n30 -type contact region 2, a p+-type contact region 3, n-region 4 in the surface voltage-sustaining region, an anode A, and a cathode K.
The surface voltage-sustaining region is located from the abscissa x=0 to x=L. The solid curve in FIG. 1(b) represents the optimum charge density D of the surface impurities for a maximum breakdown voltage under a given surface distance. In this figure, D0=qNBWpp, where q is the electron charge, Wpp is the depletion width under the same breakdown voltage of a one-sided abrupt parallel-plane junction (n+-p−) with the same substrate doping concentration, and NB is the acceptor concentration of the substrate. Thus, D0 is the charge density of the depletion layer in n+-region. The solid curve in this figure shows a case of L=2Wpp, which corresponds to the case of a breakdown voltage of the surface voltage-sustaining region being 95% of that of the one-sided abrupt parallel-plane junction with the same substrate. Dashed lines 5, 6 and 7 represent three uniform surface charges to replace the solid curve and can be used to achieve a breakdown voltage very close to that of the latter. FIG. 1(c) and FIG. 1(d) represent the lateral field profile Ex and surface potential V given by the optimum variation lateral doping under the breakdown voltage, where Ecrit and VBpp stand for the maximum field and the breakdown voltage of the one-sided abrupt parallel-plane junction with the same substrate. FIG. 1(e) shows schematically a top view of a structure to implement three sections of uniform surface impurity charge densities. There is a uniform density of donor density in n-region 4, this charge density is higher than that of the dashed line 5. There is a thin p-type region 8, with uniform acceptor density covered partly on the top of the n-region 4. From the left part to the right part, the proportion of covered n-region changes from zero to 100%. In the part containing points A and A′ in the figure, the resultant effect of the charge of the donor density of region 4 and the charge of the acceptor density of region 8 approximately equals to the average charge density of dashed line 5 in FIG. 1(b). In the right part of the part containing the points A and A′, the resultant effective charge density equals to the dash line 6. In the rightmost part of the figure, the resultant effective charge density is equivalent to dashed line 7.
It should be noted that the more number of sections used, the closer the breakdown voltage is to the value corresponding to the result of the solid curve of FIG. 1(b).
However, it can be encountered sometimes that the dose of p-region 8 and/or the dose of n-region 4 is not available in a specific CMOS or BiCMOS technology. Besides, in the case of sub-micron technology, the n-type drift region 4 is very thin, accompanying a high donor concentration and thus a low mobility. Therefore, a lateral n-MOST using such technique may have very large specific on-resistance. Moreover, as pointed in X. B. Chen, “Lateral high-voltage devices using an optimized variation lateral doping,” Int. J. Electronics. Vol. 83, No. 3, pp. 449-459 (1996), in the method of the offset of the effects of two regions with the opposite charges, there is a field parallel to the semiconductor surface and orthogonal to the edge of the stripe at point A and point A′ in FIG. 1(e), this field causes a decrease of the breakdown voltage.